In some applications, steel tubing is subjected to severe stresses, and where a series of lengths of pipe or tubing have to be coupled together, the connection or coupling itself must be able to bear the applied stresses. For example, for use as oil well casing, such tubing may be used in conjunction with steam injection into the well where temperatures of the order of 650 degrees Fahrenheit may be reached. This subjects the tubing to compressive and tensile axial loads approaching or even exceeding the actual yield strength of the material in the pipe body. Thus, any connection or coupling for joining together successive lengths of pipe must be able to withstand the axial loading without failure and still be resistant to leakage from internal pressures approaching the actual yield strength of the pipe body. As the pipe is alternately heated and cooled, the axial loading on the pipe and couplings may become alternately compressive and tensile, and throughout the coupling must maintain its seal with the pipe ends in resisting internal pressure.
Such couplings (connections) comprise a male and mating female coupling component. The male component is a suitably configured threaded portion at one end of the steel pipe, constituting the pin member of the coupling. A mating annular female component long enough in the axial direction to receive the pin ends of two adjoining lengths of pipe is internally configured and threaded at each end for mating engagement with the pin member, thereby completing the coupling. The annular female element is often referred to as the box element or box member of the coupling. Alternatively, one end of the pipe could be upset and internally threaded to constitute the female component of the connection.
Conventionally, the pin member of the coupling or connection is tapered inwardly from the proximal end of the threaded portion to the distal end to mate with a similarly tapered female threaded member of the coupling. The taper facilitates entry of the pin member into the box member.
In pipe couplings, a seal is typically maintained. The seal may be effected between the mating threaded portions of the pin member and the box member of the coupling, but this kind of seal is subject to ready leakage. In other couplings, some auxiliary sealing element (e.g. an annular elastomer) is provided. In yet other couplings of which the present invention is a species, the axial load-bearing threaded portions of the coupling do not themselves necessarily provide a seal (although they may do so); the seal relied upon is a separate metal-to-metal seal provided adjacent the axial load-bearing threaded portions, in both the pin and the box members of the coupling, or male and female elements of the connection.
The threaded axial load-bearing portion of the coupling (or connection) should conform to certain known design principles. The total axial bearing surface provided by the full-depth load-bearing threads should be at least equal to the cross-sectional area of the pipe material. The angle of orientation of the stab flanks of the axial load-bearing threads should differ from the angle of orientation of the load flanks by not less than about 15 degrees. The step height of the axial load bearing threads should be greater than the thread radius so as to avoid galling (metal abrasion). Excessive tilt of the distal end of the pin that causes the yield strength of the steel to be exceeded is generally to be avoided. Other general principles of thread design and coupling design will be known to those skilled in such design work and should be applied to the design of the connection of the present invention.
Couplings are known in which the angle of orientation of the load flank of the axial load-bearing threaded portion is negative.
Couplings are further known in which a metal-to-metal seal is provided. For best results, such surfaces should have a controlled degree of roughness, as by shot peening, grit blasting, glass bead peening, or helical microgroove threads having a pitch very small relative to the pitch of the load-bearing coupling threads.
Problems exist in known pipe couplings. Especially, the pin end of the pipe is often subjected to rough handling with consequent damage to coupling and sealing surfaces. A pipe coupling should be able to function effectively if there has been slight damage to the pin. This could be accomplished if the sealing force were designed to be above some predetermined minimum throughout the effective sealing area, and if it were designed to be well above the minimum value at each end of the sealing area. Then, if either end (or some point in between) of the sealing area were slightly damaged, the seal at the other end of the sealing area would provide an adequate seal. But many prior couplings having metal-to-metal sealing areas adjacent the threaded coupling areas typically provide a minimum or even less than minimum design sealing force at one end of the coupling and a maximum (above design sealing force) at the other. Some previous designers appear to have assumed that a knife-edge seal of sufficiently high force will suffice. That assumption tends not to be true in high-pressure high-temperature applications, nor is it valid if the sealing surface is damaged at the sealing edge. In other words, if that part of the sealing area in which the sealing force is designed to be maximum is damaged, the effective overall effective sealing force may be only the design minimum sealing force, or conceivably even less than the design minimum. This may be insufficient to prevent leakage under extreme operating conditions.